1. Field of the Invention
This invention relates to a process of afterburning combustible constituents of exhaust gases from rotary kilns used for metallurgical processes, wherein the exhaust gases from the rotary kiln are afterburnt in an afterburning chamber supplied with oxygen-containing gases.
2. Discussion of Prior Art
When metallurgical processes such as the magnetizing roasting of iron ore, the burning of limestone and particularly the production of sponge iron by a direct reduction of iron ores or the waeltz process, are carried out in a rotary kiln, the exhaust gases from the rotary kiln still contain combustible constituents, which cannot be burned in the rotary kiln. Depending on the process employed, the combustible constituents consist mainly of CO, H.sub.2, hydrocarbons, solid carbon that has been entrained by the gas or oxidizable products of the process, such as zinc or or lead vapor. The contents of combustible constituents are relatively high if the rotary kiln is charged with coals which have a high content of volatile constituents. The exhaust gases also contain incombustible fine dusts. In order to avoid a pollution of the environment and to utilize the latent heat content of the exhaust gases, their combustible constituents must be afterburnt and the dusts must be removed from the exhaust gas.
It is known from U.S. Pat. No. 2,112,566 to completely afterburn the entire exhaust gas from a rotary kiln or part of said exhaust gas by an addition of air in one stage and to pass the afterburnt gas through a bed of solids carried by a traveling grate. By the complete afterburning in a single stage, the gas is heated to such a high temperature that the lining of the afterburner chamber is subjected to a strong thermal attack and the melting or softening point of the fine dust is reached to the extent that incrustation results.
It is known from German No. 20 01 563 to provide an exhaust gas duct which has a cooled pipe wall and directly succeeds the gas outlet end of a rotary kiln and opens into the radiant heating space of a waste heat boiler. Shortly before the radiant heating space, oxygen-containing gases are supplied to effect a complete afterburning. Part of the gas which has been completely afterburnt is cooled in the waste heat boiler below the caking temperature of its solid contents and is then withdrawn from the waste boiler and used to heat the charge on a traveling grate. Whereas incrustation can be avoided to a high degree by this practice, a closely succeeding waste heat boiler is always required which involves a considerable expenditure. Additionally, care must be taken to maintain a laminar flow in the radiant heating space in order to prevent an incrustation by softened solids reaching the wall.